1. Field of the Invention
The present invention relates to a content transmission method, and more particularly, to a content transmission method enabling user devices in a home network to share content more conveniently and safely.
2. Description of the Related Art
Digital content is transmitted to a user by a content provider. The content should be protected such that only after the user obtains a proper right by paying a fee for the content, the user can use the digital content and if the proper right is not obtained, the user cannot use the content.
In order to prevent an unauthorized user from obtaining the content, the content should be encrypted with a content key and the content key should be distributed only to an authorized user.
Meanwhile, with the latest development of home network technologies, one user can own more than one user device, and also, the movement of content between devices becomes possible. Sometimes, a user wants to use content in all devices owned by the user, with one payment. However, if the content can be moved freely in a reproducible way between devices, an unauthorized user can obtain and use the content. Accordingly, in home networking, a technology which permits a movement of content between user devices in a home network of an authorized user while preventing an unauthorized user from obtaining the content or using the content even if the content obtained, is needed.
In particular, Federal Communications Commission (FCC) of the U.S. requires from July 2005, a technology by which a 1-bit broadcast flag (BF) should be inserted into a high density (HD) level content broadcast through digital broadcasting in the U.S. so that if the BF of the content is 1, the content can be protected such that an unauthorized user cannot use the content.
FIG. 1 is a diagram showing the structure of a home network.
A content provider CP transmits content to a home server HS through a transmission channel 10.
The home server HS is connected to user devices DA, DB, and DC, and manages joining and secession of an authorized user device, and transmits the content only to an authorized user device, that is, a device currently registered.
Here, a domain refers to a set of user devices connected to one home server. The devices in a domain will vary due to user devices joining or leaving the home network. Accordingly, a process for preventing a user device that has left and is no longer registered with the home network from obtaining content is needed.
Content is encrypted by using a content key and the content key is encrypted by using a common key, and the content and the content key are transmitted to each of the user devices DA, DB, and DC. The common key can be obtained only by a user device that is currently registered in the home network.
FIG. 2 is a flowchart of the operations performed by the conventional method for transmitting content.
In operation 210, the home server HS receives content C from the transmission channel 10, and by encrypting the content C using a content key Kc, the home server HS generates an encrypted content E(Kc, C). The transmission channel 10 can be any of a variety of channels, such as the Internet, ground wave, and satellite broadcasting.
In operation 220, the user device DA transmits its unique information Xa′ to the home server HS.
In operation 230, the home server HS generates a common key Ks by using the unique information Xa′, and then transmits it to the user device DA.
In operation 240, the home server HS transmits the encrypted content E(Kc, C), an encrypted content key E(Ks, Kc), and a license L_A to user device DA.
The license L_A includes usage rule UR on the content C and the unique information Xa′ of the user device DA.
In operation 250, the user device DA extracts the unique information Xa′ from the license L_A received in operation 240, and then compares this with DA's own unique information Xa.
In operation 260, the user device DA determines if Xa′=Xa and whether or not it is possible to generate the content key Kc by decoding the encrypted content key E(Ks, Kc) received in operation 240 by using the common key Ks received in operation 230. If these two conditions are met, the user device DA in operation 220 is the same as the user device DA in operation 240, and therefore the user device DA can reproduce the content C.
However, the conventional method has the following drawbacks.
First, since the content C is bound to a user device, sharing the content in two or more user devices belonging to the same user is inconvenient. All these devices, that is, even devices belonging to the same user, must receive newly issued licenses based on their respective unique information. For example, when the device DB tries to reproduce the content received from the device DA, operations for transmitting unique information Xb′ of the device DB, generating content Kc′, generating common key Ks′, and generating license L_B for the device DB must all be performed again.
Secondly, unique information of a user device is exposed to external devices.
Since most of the unique information Xa′, Xb′, . . . , of user devices play important roles in the security aspect, it is preferable to avoid leakage of this information.
Thirdly, sharing content between user devices should be performed through the home server HS.
Generally, since authorization of use of the content C relies, in most cases, on a fee payment by a user and one user wants to use freely content in two or more user devices belonging to the domain of the user, the drawbacks described above become more serious obstacles to the development of home networks, the demand for which is currently increasing.